Methods for the detection of a cell (e.g., a pathogenic microorganism or a cancer cell) in a sample often involve the detection of a biological activity (e.g., an enzyme activity or a biochemical pathway) known to be associated with the particular cell. Often, the biological activity is detected using an indicator system that is changed via the biological activity to a biological derivative.
Some methods employ two indicator systems to detect a particular type of cell. For example, methods to detect E. coli can include a first indicator system that includes lactose in combination with a pH indicator. The fermentation of lactose to organic acids indicates the presence of a member of the coliform bacteria (which includes E. coli and other enteric microorganisms). The methods also include a second indicator system, such as 4-methylumbelliferyl-β-D-glucuronic acid, which is used to detect the enzyme β-glucuronidase, an enzyme found in most E. coli. Thus, in a method employing both indicator systems, the accumulation of acidic end products from lactose, along with the accumulation of a fluorescent compound (4-methylumbelliferone) can indicate the presence of E. coli in a sample.
The detection of a particular biological activity in a sample may be indicative of viable cells in the sample. Bacterial spores, for example, include biological activities (e.g., enzyme activities such as α-glucopyranosidase or β-glucopyranosidase) that may be used in methods (e.g., including rapid methods) detect the presence of viable spores in a sample. Destruction of one of these or other biological activities can be used to verify and/or validate the efficacy of a sterilization process.